1. Field of the Invention
This invention relates to paper transport devices applicable to document scanners, copying machines, printers, facsimile machines and like apparatus. More particularly, the invention pertains to paper transport devices in which paper, such as an original document or copying paper, is transported in a specified direction by at least one pair of rollers.
2. Description of the Background Art
Referring to FIG. 1, a conventional image forming apparatus, such as a copying machine or a facsimile machine, typically comprises a document tray 610 provided in an upper part of a main body 600, a document feeder 620 which is provided at the right of the document tray 610 (as illustrated in FIG. 1) and has a top cover 621, a scanner 630 provided beneath the document tray 610, an optical unit 640 provided beneath the scanner 630, an image forming unit 650 which is disposed beneath the optical unit 640 and includes a photosensitive drum 651 which produces a toner image from an original image exposed by laser light emitted from the optical unit 640 and transfers the toner image onto paper, a paper feed section 660 which is disposed beneath the image forming unit 650 and includes a paper cassette 661, a fuser assembly 670 provided downstream of the paper feed section 660, and a paper output section 680 provided downstream of the fuser assembly 670, for example.
The original image is read, or scanned, in the following manner. An original document loaded on the document tray 610 is first transferred rearward (rightward as illustrated) as it is successively nipped by a plurality of roller pairs of the document feeder 620. Then, the document feeder 620 reverses the feed direction of the document and ejects it back onto the document tray 610. The scanner 630 scans the original image while the document is being transferred by the document feeder 620.
If the document jams as it is transferred rearward by the document feeder 620 in the image forming apparatus thus constructed, it is necessary to open the cover 621 and remove the document from between the roller pairs located in an upper part of the document feeder 620. If the document feeder 620 employs a multiple sheet feed assembly in which each successive sheet is separated and transported from multiple sheets of an original document stacked on the document tray 610, for example, a jam could occur while a sheet is being transferred toward the rear side of the apparatus. Explained below is how this type of jam has conventionally been dealt with.
FIGS. 2 to 4 are diagrams showing a principal portion (multiple sheet feed assembly) of a conventional paper transport device. This paper transport mechanism is constructed such that a roller 643f rotatably mounted in a roller housing 647 can project outward and retract inward through an opening 647b formed in an upper surface of the roller housing 647. In this construction, a cylindrical outer surface of the roller 643f is caused to partly protrude upward and push against another roller located above (not shown) to thereby grip and transport a sheet of paper. In the event of a paper jam, the roller 643f is retracted into the roller housing 647 so that the roller 643f is separated from the upper roller, making it possible to remove a sheet which has been seized between the rollers. The construction of this paper transport device is now described in further detail referring to FIGS. 2 to 4.
The roller housing 647 has a pair of side plates 647a attached to both ends, and a cover 648 which can be opened and closed is fitted to pivot pins 645a provided on both side plates 647a.
As depicted in detail in FIGS. 3 and 4, the roller 643f is mounted on a roller shaft SF1 which is fitted in vertically extending slots 647c formed in the side plates 647a attached to the opposite ends of the roller housing 647 so that the roller shaft SF1 can move up and down. When the cover 648 is swung up about the pivot pins 645a, the cylindrical outer surface of the roller 643f which has partly protruded from the cover 648 through its opening 647b is fully retracted into the inner space of the roller housing 647. More particularly, L-shaped levers 647e are provided inside the individual side plates 647a which are attached to the roller housing 647, each L-shaped lever 647e being fitted swingably about a pivot pin 647d which is formed at an upper rear part of the corresponding side plate 647a, projecting from its inside surface, with one end of the L-shaped lever 647e extending toward the roller 643f and the other end directed downward. Each end of the roller shaft SF1 is fitted in a hole 647f formed in one terminal portion of the L-shaped lever 647e that extends toward the roller 643f, while a spring 649 is hooked between a projecting pin 647g formed at the other terminal portion of the L-shaped lever 647e and a pin 648d formed on a fixing plate 648c, projecting inward from the inside surface of a rear part of the cover 648. The roller shaft SF1 fitted in the holes 647f formed in both side plates 647a is biased upward by springs 647p disposed below the roller shaft SF1 at both ends thereof.
In this construction, when the cover 648 is closed as shown in FIG. 3 (i.e., when the paper transport device is ready to feed the original document), the roller shaft SF1 is pushed upward by the springs 647p and, therefore, the roller 643f is lifted and part of its cylindrical outer surface is exposed to the outside of the roller housing 647. On the other hand, when the cover 648 is swung up about the pivot pins 645a as shown in FIG. 4, each fixing plate 648c of the cover 648 shifts rearward so that each spring 649 is pulled rearward and, thus, the downward-directed end (rear end) of each L-shaped lever 647e swings clockwise about the pivot pin 647d (as illustrated in FIG. 4), overwhelming a pushing force exerted by the relevant spring 647p. As a consequence, the roller 643f is forced downward and the part of its cylindrical outer surface which has been exposed to the outside of the roller housing 647 is retracted into its inner space. Since the roller 643f is separated from its upper roller at this point, an operator can remove the sheet of the document jammed between the two rollers.
The paper transport device of the above-described construction is provided with complicated mechanisms on both sides of the roller 643f for moving the roller shaft SF1 upward and downward in interlocked action with the opening and closing of the cover 648. The result of this construction is an increase in overall costs of the paper transport device, and this is one of the factors of cost increase of the conventional image forming apparatus incorporating this type of paper transport device.
The foregoing discussion has focused on how a jam which occurs while the document feeder 620 is transferring a sheet of the original document toward the rear side of the apparatus is dealt with. Explained next is how a jam which occurs while a sheet of the document is being transferred forward is dealt with. When this type of jam has occurred, it is necessary to swing up the document tray 610 about its one side to expose a bottom part of the document feeder 620 and then remove the sheet which has jammed from between a pair of rollers located in the bottom part of the document feeder 620. From this, it is recognized that the conventional paper transport device has required a very complicated procedure for dealing with this type of jam.
Although rollers of each roller pair are held in firm contact with each other in normal paper transport conditions, it is desirable to relieve the rollers of their firm contact when dealing with a jam. Then, after removing a sheet which has jammed in the paper transport device, the rollers of each roller pair must be brought back to firm contact again. Operations for relieving and reestablishing the firm contact have been so complicated because such operations should be performed for each individual roller pair.
It is to be understood that the aforementioned problems are not limited to paper transport devices for transporting an original document but would also occur in various paper transport devices for feeding copying paper, printing paper, or other types of paper.